Solid-state NMR studies of PEO in various environments: Conformation, chemical shift, and dynamics

by Harris, Douglas Jeffrey

Abstract (Summary)

Double-quantum NMR spectroscopy was employed to determine the torsion angle distributions of the OC-CO bonds in poly(ethylene oxide), PEO, in various environments: crystalline PEO, PEO-p -nitrophenol (PEO/PNP) and PEO-resorcinol molecular complexes, PEO nanocomposites with a smectite clay and MoS2 , and PEO-poly(Ã?Â±,L-glutamate) blends. The technique not only allowed quantitative determination of both the relative ratios and average torsion angles for the trans and gauche bonds, but also set an upper limit for the width of the torsion angle distribution. The studies confirmed that the OC-CO bonds in crystalline PEO all adopt a gauche torsion angle and determined an average angle of Ã?Â¨ = 74 Ã?Â± 4Ã?Â° with a distribution width of Ã?Ã?Â¨ < 9Ã?Â°. The spectra of the PEO-resorcinol and PEO-poly(Ã?Â±,L-glutamate) blends showed little difference from that of crystalline PEO. PEO/PNP was shown to be a suitable model system for studying conformation because 1/3 of OC-CO bonds are trans and the complex has extremely slow dynamics at room temperature. Simulations of the double-quantum spectra set an upper limit for the trans torsion angle distribution width of Ã? Ã?Â¨ < 7Ã?Â°. Finally, the OC-CO bonds of the PEO chains in the nanocomposites were found to be 90 Ã?Â± 5% gauche, which provides valuable constraints on the possible chain conformation in the intercalation gaps. The effects of packing, hydrogen bonding, and conformation on 13 C chemical shift were studied in crystalline PEO and the PEO/PNP complexes. CP-MAS spectra of crystalline PEO at 200 K, acquired under strong 1 H decoupling and after filters to suppress the mobile components, resolved four distinct peaks. Deconvolution of the 1D spectrum showed chemical shifts ranging from 74.7 to 71.6 ppm for the 14 carbons in the 72 helical repeat unit. Dynamics of the conductive phase was studied in polymer electrolytes and block copolymers from poly(oligo(oxyethylene) methacrylate) macromonomer (POEM), lauryl methacrylate, n -butyl methacrylate, and methyl methacrylate. Enhanced conductivity of diblock copolymer electrolytes with a low Tg non-conductive phase is at least partially due to faster chain dynamics in the conductive phase. A higher Tg non-conductive block shifts the observed dynamics curves towards a higher temperature by approximately 5Ã?Â°C. The dynamic inhomogeneity in the POEM side chain was determined by wideline separation experiments to be on a length-scale of less than 5 nm. (Abstract shortened by UMI.)